At the transition of an electric field from a first medium to a second medium, electric stresses harmful to the electric equipment can ensue due to a discontinuity in the electric field. In a shielded high-voltage cable for instance, the electric field is uniform along the cable axis and there is variation in the field only in the radial direction. When the cable is terminated or spliced, the shield of the cable is removed for a distance along the cable. The removal of the shield causes a discontinuity in the electric field at the shield end, resulting in high electric stresses. These high stresses must be reduced in order not to impair the expected life of the system.
The electric stresses in question can be reduced by grading the electric field at the transition of the electric field from the first medium to the second medium, e.g. from a shielded cable part to a cable part where the original shield has been removed. A number of methods have been developed and employed for this kind of field grading. The present invention is related to so called resistive and capacitive field grading.
The resistive field grading can be used in alternating current as well as direct current applications. Resistive field grading can also be used in order to achieve field grading when voltages are occurring in the form of impulses. In case of a cable ending of the kind indicated above, a body having a suitable resistance is introduced around the unshielded part of the cable in the area closest to the shielded part of the cable and in electric contact with the shield. When a positive voltage is applied across the cable a current flows through the body towards the shield of the cable, which shield is at earth potential. A resistive voltage drop then occurs in the body, which results in a more uniform distribution of the potential. This potential distribution will be more linear if the body consists of a material exhibiting a non-linear electrical resistance that decreases with an increasing electric field. The closer to the edge of the shield, the higher the electric field in the field grading body and, consequently, the lower the electrical resistance in the body if the body exhibits such a nonlinear electrical resistance. In this way, the voltage drop along the field grading body will become more uniformly distributed in a body that exhibits such a non-linear electrical resistance than in a body that does not.
The capacitive field grading is used in alternating current applications. Capacitive field grading can also be used in order to achieve field grading when voltages are occurring in the form of impulses. In case of a cable ending of the kind indicated above, a body of a material having a dielectric constant higher than that of the insulation and as low losses as possible is introduced around the unshielded part of the cable in the area closest to the shielded part of the cable and in electric contact with the shield, whereby a spreading of the equipotential lines will be achieved. Capacitive field grading properties are also desired in a material adapted for grading the electric field in high-voltage direct current applications so as to achieve an effective field grading in case of suddenly occurring voltage surges.
Polymers play an important role in electrical insulating and field grading technology because of their high electrical strength, ease of fabrication, low cost and simple maintenance. Conventionally, additives have been mixed into polymer matrices to improve their resistance to degradation, to modify mechanical and thermomechanical properties, and to improve electrical properties such as high-field stability. One limitation of conventional additives is the negative effect they can have on electrical properties. In the ideal case, an additive will both modify the property of interest and improve other properties, or at least not degrade the other properties. Nanocomposite structures composed of nanostructured fillers homogeneously or heterogeneously mixed with a polymer matrix are described in U.S. Pat. No. 6,228,904. Crosslinked conducting polymer composites including a major phase and a minor phase containing conducting filler such as carbon black, graphite, metallic particles, conducting polymers, carbon fiber, fullerenes, and/or carbon nanotubes dispersed in a semicrystalline polymer are disclosed in U.S. Pat. No. 6,417,265. Neither of the above patents mention field grading or insulating applications. WO 2004/038735 describes a field grading material consisting of a polymeric matrix containing a nanoparticle filler, and devices utilizing such materials. Neither surface treatment nor non-uniform distribution of the particles is mentioned. JP 11086634 relates to an insulating material wherein micron-sized magnesium oxide particles were surface treated with vinylsilane and mixed with ethylene homopolymers or copolymers. WO 2004/034409 discloses a nanometric composite including a stoichiometric nanoparticulate filler embedded in a polymer matrix with enhanced electric strength and improved voltage endurance properties. Non-uniform distribution of the filler is not described. However, there remains a continuing need for polymer composites having an enhanced balance of properties, particularly for use in these technology areas.